A new rhodamine based chemodosimeter for Ni²⁺ with high sensitivity and selectivity

Abstract

A new rhodamine derivative by modifying the ortho-position of the carboxylate in benzolate with amino pyridine has been developed, which exhibits high sensitivity and selectivity toward Ni²⁺ with a detection limit down to 4.6 ppb.

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Nickel (Ni) is widely used in various industrial applications such as in electroplating, Ni–Cd batteries, pigments for paints, ceramics, catalysts for hydrogenation, surgical and dental prostheses and as magnetic tapes for computers. Besides, the Ni²⁺ is an essential cofactor for a variety of enzymes that play important roles in microorganisms and plants, particularly in energy and nitrogen metabolism. However, excessive Ni²⁺ in our body will result in adverse health effects ranging from allergic dermatitis to lung and nasal sinus cancers.¹ Therefore, the detection of Ni²⁺ is very important. Up to now, various methods have been recently reported for the determination of Ni²⁺ concentration in different biological, industrial and food samples. The mature methods to detect Ni²⁺ are mostly based on time consuming and sophisticated analytical techniques, such as atomic absorption/emission spectrometry, liquid chromatography and voltammetry.² Although these methods are accurate, they are not suitable for convenient “in-the-field” detection as they normally require expensive instruments and sample pretreatment, and also have serious influence by the interference of coexisting ions. Therefore, it is very important to develop sensitive, rapid, and simple-to-use methods to sense Ni²⁺. Meanwhile, colorimetric sensors have also attracted much attention, because the detection can be analyzed by the naked eye. It also allows on-site and real-time detection in an uncomplicated and inexpensive manner, providing qualitative and quantitative information.³ Unfortunately, colorimetric sensors for Ni²⁺ with high selectivity and sensitivity are rare.⁴

Rhodamine B is a widely used organic dye due to their excellent photophysical properties, such as high extinction coefficients, excellent quantum yields, and relatively long emission wavelengths.⁵ But the partial tautomerization from lactone form to zwitterionic form in polar solvent leads to the strong background signal of rhodamine B, which limits its application in chemosensors (Fig. 1a and S1, ESI†).⁶ But since Czarnik and the co-worker reported the first modified-rhodamine for detecting Cu²⁺ in 1997, many rhodamine-based probes have been developed.⁷ However, the modification in these probes are usually on 2′ position (Rhodamine 2′, e.g., the oxygen atom is replaced by nitrogen atom or sulfur atom) and they can only detect some common cations (such as Cu²⁺, Hg²⁺, Fe³⁺, Cr³⁺ and so on) (Fig. 1b left).⁸ To our best knowledge, the highly sensitive and selective rhodamine-based probe for Ni²⁺ hasn't been developed yet. The probable reason is that the present Rhodamine 2′ could not well chelate Ni²⁺ and some new modifications on the skeleton of rhodamine seem particularly important. Here, we put forward a new modification strategy on rhodamine by introducing some proper ligands to ortho-position of carboxylate group in rhodamine (Rhodamine 3′, Fig. 1b right) and they would exhibit several advantages: (1) Y atom and R group in the molecular skeleton will be quite variable and they could be expected to detect different analytes by suitable combinations of Y atom and R group; (2) this modification could well maintain carboxylate group to participate in the chelation and finally enhance the recognition ability. As a prototype, we designed and synthesized a new ortho-modified rhodamine derivative of Rha-py (Fig. 1c). As we expected, the residual carboxylate group participates in the chelation of Ni²⁺ with other ligands and finally realizes the detection of Ni²⁺ with high selectivity and sensitivity.

Fig. 1 (a) The equilibrium of rhodamine B between neutral lactone form and zwitterion form; (b) rhodamine structures with modification on 2′ (Rhodamine 2′) and 3′ (Rhodamine 3′) positions; (c) molecular structure of Rha-py and a proposed mechanism for Rha-py in sensing Ni²⁺. Change in color of Rha-py (4 μM) upon addition of Ni²⁺ (40 μM) under ambient light.

Rha-py was facilely synthesized in high yield with only three steps. Detailed synthesis was given in ESI.† We first tested the optical properties of Rha-py in aqueous solution. Rha-py showed a strong absorption at 550 nm in pure water (Fig. S1†), indicating that Rha-py partially existed in zwitterion form.⁹ And the fluorescence of the probe was weak, which is presumably caused by the effect of photo-induced electron transfer (PET) from adjacent electron-donating amine on its benzyl carboxylate structure.¹⁰ In the meanwhile, the thermal dissipation of excitation energy of the probing molecule by strong H bonding with water is another reason for the weak fluorescence. Because water has both the high polarity and H-bond donating and accepting properties, which will effectively decrease capability of coordination from its three built-in functional groups of amine, pyridine and carboxylate, metal ions are not easy to be fixed by these coordination groups in pure water.

Therefore, further evaluation of the Rha-py was then conducted in various mixed solutions of water and CH₃CN, and results indicate that a non-negligible absorption for ring-opened form of Rha-py still exist in neutral condition if the percentage of CH₃CN in solution is less than 50%. CH₃CN–H₂O (v/v = 9 : 1) is finally selected as an ideal test solvent to avoid the background absorption (Fig. S1, ESI†). The absorbance at 550 nm was about 40-fold enhanced in pH range from 4.5 to 7.5 (in HEPES buffer) in presence of 10 equivalents Ni²⁺ (Fig. S2, ESI†), showing obvious pink color from colorless solution. Detailed sensing capability of this probe toward various cations were systematically investigated thereafter with UV-Vis spectroscopy in CH₃CN–H₂O solution (v/v = 9 : 1) with HEPES buffer at pH = 7.1.

Tested cations include Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Mn²⁺, Ag⁺, Hg²⁺, Al³⁺, Cr³⁺, Fe³⁺, Fe²⁺ and the counter ions are NO₃⁻ or Cl⁻. When 10 equivalents of these cations (c = 40 μM) were added to the test solution of Rha-py (c = 4 μM) at room temperature, the solutions remained colorless respectively, except for the Ni²⁺, Co²⁺ and Cu²⁺. Upon addition of Ni²⁺, a dramatic pink color was observed by the naked eye immediately. In corresponding UV-Vis spectra, a new absorption band peaked at about 550 nm was formed herein, which is in good agreement with its color change on Ni²⁺ (Fig. 2). Furthermore, the pink color from this Ni²⁺ sensing still remained after a day. Co²⁺ also resulted in a very weak increment in absorption due possible to the similarity in size with Ni²⁺. Although, Cu²⁺ also induced ring-opening reaction initially with the appearance of pink color, the pink color however disappeared quickly in ten minutes (Fig. 3), but the pink color kept for a rather long time in presence of Ni²⁺ (Fig. S8, ESI†). In order to find out the reason, LC-MS experiments were carried out (Fig. S11, ESI†), the retention time of Rha-py was 1.8 min with its mass signal at 549.3, and the retention time of Rha-amino (amino group on the 3′ position of rhodamine B) was 2.7 min with its mass signal at 458.2. However, in the mixture of Cu²⁺ and Rha-py, no signal of Rha-py or its metal complex with Cu²⁺ was found, but the signal of Rha-amino. We presumed Rha-py could coordinate with Cu²⁺ well, but Cu²⁺ will catalyze its degradation to generate Rha-amino. Rha-amino cannot fully coordinate with Cu²⁺ and it exists in colorless lactone form.¹¹ This distinct difference on color fading time will help to differentiate these two interfering cations in practical sensing. In corresponding fluorescence spectra, Ni²⁺ induced about 5-fold enhancement (Fig. S3, ESI†), and Cu²⁺ quenched the fluorescence of the probe, and we presumed it is because their electron configuration of d-orbital and the coordinating effect are different.

Fig. 2 Absorption spectra of Rha-py (4 μM) in CH₃CN buffered with HEPES (9/1, v/v, pH = 7.1, 20 mM) after the addition of different metal ions (40 μM) in 3 min, except for copper ion which was detected after 18 min. Metal ions: Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Mn²⁺, Ag⁺, Hg²⁺, Al³⁺, Cr³⁺, Fe³⁺, Fe²⁺.

Fig. 3 Gradual color change of the solution of Rha-py (4 μM) after addition of 10 equivalents of various metal ions in CH₃CN buffered with HEPES (9/1, v/v, pH = 7.1, 20 mM). Pictures above represent the change with the time in clockwise direction. Respectively, in each picture, the cube containing Rha-py and different cations (blank, Co²⁺, Ni²⁺, Cu²⁺, others, from left to right).

An important feature of a sensor is its selectivity toward the analyte relative to other competitive species. Therefore, competition experiments were carried out by adding Ni²⁺ (c = 40 μM) to a solution of Rha-py (c = 4 μM) in the presence of miscellaneous cations (c = 40 μM), respectively. These miscellaneous competitive ions did not induce significant absorption changes of Rha-py in the absence of Ni²⁺, except Cu²⁺, which caused the color change at first and then the color faded in minutes. Upon addition of Ni²⁺ to the above solutions, pink color appeared except for the one with Cu²⁺ in it, which didn't show further color change (Fig. 4). These results revealed that Rha-py had a remarkable selectivity toward Ni²⁺ with the only interference of Cu²⁺, however, by monitoring the time-resolved spectra or color fading rate, Cu²⁺ can be differentiated.

Fig. 4 Absorption spectra bar of Rha-py (4 μM) in the presence of 10 equivalents of Ni²⁺ with 10 equivalents of various metal ions in CH₃CN buffered with HEPES (9/1, v/v, pH = 7.1, 20 mM). The spectra were measured 3 min after the addition of the metal ions, except Cu²⁺ was detected 18 min after the addition. Bars represent absorbance at 550 nm. The black bars represent the addition of the competing metal ions to the solution of Rha-py. The red bars represent the addition of competing metal ions and Ni²⁺ to the solution of Rha-py: 1, blank; 2, Ag⁺; 3, Al³⁺; 4, Ca²⁺; 5, Cd²⁺; 6, Co²⁺; 7, Cr³⁺; 8, Cu²⁺; 9, Fe²⁺; 10, Fe³⁺; 11, Hg²⁺; 12, K⁺; 13, Mg²⁺; 14, Mn²⁺; 15, Na⁺; 16, Zn²⁺; 17, Ni²⁺.

To identify the stoichiometry between Ni²⁺ and Rha-py, Job's plot had been drawn (Fig. S4, ESI†). We can find when the molar fraction of Ni²⁺ was 0.5, the absorbance at 550 nm reached a maximum, indicating the formation of a 1 : 1 complex between Rha-py and Ni²⁺. And the association constant (K_a) of Rha-py with Ni²⁺ was determined as 4.87 × 10⁴ M⁻¹ using the Benesi–Hildebrand equation (Fig. S6, ESI†).^12a The spectroscopic detection limit for Ni²⁺ was 7.8 × 10⁻⁸ M (Fig. S7, ESI†), calculated with the equation of 3σ/S.^12b Which is lower than the upper limit 0.04 mg L⁻¹ (6.8 × 10⁻⁷ M) recommended by the U. S. Environmental Protection Agency (EPA) for drinking water.¹³

The interaction mechanism of Ni²⁺ with Rha-py was further studied with infrared spectroscopy (IR) and MALDI-TOF experiments. From the IR spectra (Fig. S9, ESI†), the stretching vibration absorption peaks of C=O and vibration of pyridine have a significant shift towards lower wavenumbers, and the vibration of carboxyl acid appeared, indicating the complexation with Ni²⁺.¹⁴ In MALDI-TOF, the number 605.34 stands for the complex of Ni²⁺ and Rha-py, which confirmed the coordination ratio was 1 : 1 (Fig. S10, ESI†).

In order to test the practicality of the Rha-py probe, we took the water from South Lake, Changchun as a sample. The sample was filtered to remove organisms and analyzed by the proposed absorption method under optimized conditions, and no Ni²⁺ is detected. Then Ni²⁺ is added to the sample on purpose. The recovery of Ni²⁺ and R.S.D. of probe Rha-py are satisfactory (Table S1 ESI†),¹⁵ indicating that the present colorimetric probe of Rha-py was applicable for the determination of Ni²⁺ in contaminated water samples.

Conclusions

In summary, we first demonstrated the modification strategy on the 3′ position of rhodamine and developed a colorimetric probe for Ni²⁺. The prototype probe of Rha-py based on the strategy shows high sensitivity and selectivity for Ni²⁺. The probe could respond to Ni²⁺ quickly. Unfortunately, Cu²⁺ is an interference for Ni²⁺ detection, however, it can be differentiated by time-resolved spectra or color fading rate. The spectroscopic detection limit is lower than the upper limit recommended by EPA for drinking water. With good performance in analysis of Ni²⁺ in lake water, we believe that this novel Ni²⁺ chemosensor can be used for monitoring Ni²⁺ in contaminated water samples with high sensitivity and selectivity. This work will be a starting point of further research, we will make further effort to modify the 3′ position along with the modification on the other position. This will surely inspire more development of chemosensors for environmental and medical applications.

Acknowledgements

This work was supported by the National Science Foundation of China (Grant No. 51373068) and program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT101713018).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹HNMR, ¹³CNMR data, LC-HRMS data, X-ray diffraction data, synthetic details and spectra. CCDC 1036431. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra11737b

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